A source of white noise is a key element in Random Number Generator (RNG) circuits, which are used widely in hardware encryption devices. White noise is a type of random noise that may be generated in electronic circuits mostly as a result of random motion of electrons at non-zero temperature. It is know to generate white noise using linear resistors as well as active devices such as diodes and transistors. A linear resistor generates white noise whose power is proportional to the resistance, so that in order to obtain high amplitude white noise, a high resistance element is required. Usually, high resistance is achieved by increasing the length to width ratio of the resistor. Increasing the dimensions of the resistor in this manner enlarges its parasitic capacitance, which in turn reduces the noise bandwidth and thus reduces the noise amplitude. Thus the parasitic capacitance acts against the increased resistance and it is known that in practice the noise amplitude is actually determined by parasitic capacitance of the resistor and the input capacitance of the load.
“Review of noise in semiconductor devices and modeling of noise in surrounding gate MOSFET” by Bipin Rajendran published by the Department of Electrical Engineering Stanford University, December 2001 presents an introduction to different types of noise associated with CMOS with particular reference to white noise and to the problems associated with thermal noise and 1/f noise, also known as excess or flicker noise. White Gaussian noise is a name for a random process with Gaussian probability distribution behavior and has a autocorrelation function of a Dirac delta function type, which means that if the process is sampled at any two or more points of time the samples will have no correlation to each other. Noise spectral density is a Fourier transform of the autocorrelation function which in the case of white noise is constant (flat) for all frequencies. Any other type of noise is not white and sometimes called “colored”. 1/f noise is not white noise because its autocorrelation function is not a Dirac delta function, and samples of a 1/f random variable have pretty strong correlation to each other.
“Prospects for charge sensitive amplifiers in scaled CMOS” (O'Connor et al.) appearing in Nuclear Instruments and Methods in Physics Research, A 480 (2002) 713725 presents a more detailed description of some specific circuit implementations for replacing the feedback resistor commonly used in noise generators by MOSFETs. Thus, a circuit is described in Section 6.4 on page 722 where the feedback resistor is replaced by a MOSFET in the triode region, which uses the gate to channel capacitance with source and drain shorted.
It thus emerges that use of MOSFETs in white noise generators that are used in the linear region to serve as very high impedance resistors is well known. It is also known that a MOSFET does not generate white noise except over a midband region and that the amount of noise generated depends on the quiescent conditions and source resistance. It thus follows that transistors require proper biasing for correct operation and this means that some DC current is always flowing through such a device. DC current is responsible for generating 1/f noise and as a result the generated noise is not white. 1/f noise is a shot noise which is not Gaussian, and in transistors, diodes and other active elements the 1/f noise is directly proportional to a DC current flowing through a device or in some cases to a powered DC current. In addition, the parasitic capacitance of the device also reduces the noise bandwidth, thus reducing the noise amplitude.
It would thus improve the quality of white noise produced by such a MOSFET if the effect of DC bias could be reduced.